Thick and wide strip material, e.g. steel strip, is reduced to the desired dimensions and its strength and flexibility are enormously increased by subjecting it to a plurality of plastic-working operations, normally by moving it in a run consisting of a multiplicity of passes between rolls that compress it with great force. Such action, combined with some tension in the strip, decreases the workpiece thickness (its smallest dimension as compared to its length which extends in the travel direction and its width which is perpendicular thereto and to the compression direction) and its width, while increasing the length of the strip workpiece. Thus the workpiece travel speed increases slightly with each pass between the rolls.
The rolling is done in roll stands each comprising two parallel and small-diameter working rolls that define a nip through which the strip passes. A pair of much larger-diameter backing rolls flanks these working rolls, each backing roll engaging the respective working roll in line contact and urging it toward the other working roll. Thus the working rolls can exert enormous pressures on the workpiece because of the small contact area between them. The natural tendency of this force to bend these small-diameter working rolls outwardly away from the workpiece is countered by the stiffer large-diameter backing rolls. In addition complex systems are provided to oppositely bend the backing and working rolls so they are convex toward the workpiece to substantially eliminate any bend in the working rolls.
The workpiece width is invariably smaller than the effective width of the rolls, which are cylindrical with stub shafts on their ends so this effective width is the length of the middle largest-diameter cylindrical portion. The contact width, that is the length of the contact zone between the workpiece and the working rolls measured parallel to the roll rotation axes which are in a plane perpendicular to the workpiece travel direction, is generally equal to slightly less than the effective width for the first pass, which may include several passages through a given roll stand, and is a small fraction of this effective width for the last pass. Thus, in addition to bending, the working rolls are subject to limited elastic deformation in the form of a flattening in the region of contact with the workpiece. The out-of-contact end portions of the two working rolls are not flattened, but are cylindrical, so the nip between the working rolls is slightly smaller to both sides of the workpiece than at the workpiece. As a result the strip edges are subjected to greater pressure and are flattened, a phenomenon known as edge drop. Thus the strip workpiece is not of uniform thickness.
To overcome this problem the above-mentioned roll-bending equipment is used to bring pressure on the working-roll ends. Obtaining enough bend to cancel out the above-described flattening of the rolls is very difficult.
In addition it is known to make the working rolls slightly barrel shaped so that when they bend their side in contact with the workpiece becomes perfectly straight. The problem with this type of arrangement is that the rolls then are only suitable for use for a limited range of workpiece widths, needing complex remachining for different sizes.
In another known system, such as described in German patent document Nos. 955,131 and 2,206,912 the working rolls are braced against outwardly tapered or barrelf-shaped intermediate backing rolls in turn backed up by cylindrical rolls. The frustoconically tapered end regions of the intermediate backing rolls start about level with the strip edges. Thus as band width changes the intermediate backing rolls must be changed also, necessitating the use of complex supports for the rolls as well as a magazine of different roll sides.
A solution to this problem has been the use of intermediate backing rolls which each have only one tapered end region and which can be moved axially in the roll stand. The edge of this end region is aligned vertically with a respective workpiece edge, and is moved in as the rolling operation progresses and the workpiece becomes narrower. Obviously the equipment that does this is extremely complex, expensive, and difficult to operate. In addition such uneven bending of the working rolls creates a workpiece of nonuniform thickness in its central regions. The working rolls in such a system also wear at an excessively fast rate.